Separation of aromatics from hydrocarbon streams



Dec. 17, 1963 R. M. BUTLER ETAL 3,114,783

SEPARATION OF AROMATICS FROM HYDROCARBON STREAMS Filed Aug. 27, 1959 45 EXTRACTIVE DISTILLATION ZONE 4g- 7 -1E XTRACTION ZONE 8 I IS FLASH TOWERL l 2 as SOLVENT 1. M RECOVERY ZONE I Roger M. Butler John A. Bichurd INVENTORS United States Patent O M 3,114,783 SEPARATION OF AROMATICS FROM HYDROCARBON STREAMS Roger M. Butler, Sarnia, Ontario, and John A. Bichard, Point Edward, Ontario, Canada, assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Aug. 27, 1959, Ser. No. 836,507 5 Claims. (Cl. 260-474) The present invention concerns a process for the separation of aromatics and parafiins from mixtures thereof. More particularly, the invention relates to a process for separating aromatics and parafiins from a hydrocarbon stream containing a mixture thereof by combining an extractive distillation step and an extraction step employing a solvent selective to the aromatics. Specifically, the invention relates to a process for separating from a hydrocarbon stream which includes light paraflins, such as C and C heavy parafiins, light aromatics, such as C and heavy aromatics, such as C and C substantially pure parafiinic and aromatic products by combining fractionation, extractive distillation and extraction in a manner not previously known in the art.

The separation of aromatics from parafiins in a hydrocarbon stream containing a mixture thereof by means of liquid or gaseous extraction has been found to be increasingly more difficult as the boiling range of the hydrocarbon stream is increased. This result is explained by the fact that the relative solubilities of the low boiling paraffins, such as C and C and the heavy aromatics, such as C and C are not widely separated. Thus, for example, when diethylene glycol-water mixtures are used to extract hydrocarbon streams which contain such a mixture of constituents, it has been found that if the solubility conditions are maintained so as to completely extract the heavy aromatics, the extract also contains appreciable quantities of low boiling parafiins. It is, therefore, desirable to remove the low boiling parafiins prior to the liquid or gaseous extraction step. One method employed to achieve this end consists of fractionating the hydrocarbon feed stream in a preparation tower so that a portion of the C and C paraflin material is taken overhead with as little benzene as possible. However, benzene and C paraffins form non-ideal solutions and it is therefore practically impossible to prevent the benzene from accompanying the C -C fraction. Such a preparation tower is operated with an overhead cut point of about 140 to 160 5., depending on the amount of C paraflins to be removed overhead versus how much benzene will be lost with this overhead fraction. The bottoms from the preparation tower are directed to an extraction zone Where they are contacted with a solvent selective to aromatics. The aromatics are removed in the extract phase, while the paraffins remain in the raifinate. The primary inadequacies of this method are three-fold. First, the C -C fraction taken overhead from the preparation tower contains benzene and thus decreases the availability of aromatics which may be recovered in the extraction zone. Secondly, this C -C fraction, because it contains benzene, is unsuitable, without further treatment, as feed to isomerization or paraffin alkylation zones which employ aluminum chloride, aluminum bromide, or other similar acid catalysts. Thirdly, considerable C parafiins are lost in the rafiinate and if 3,114,783 Patented Dec. 17, 1963 they are to be recovered therefrom for isomerization or paraffin alkylation processing furtherfractionation of the raffinate is necessary.

The above-mentioned disadvantages are overcome by the process which is the basis of the instant application. In the instant process the hydrocarbon feed. mixture is fractionated to remove all the C paraflin hydrocarbons therefrom. This C paraffin fraction, which will include most of the benzene and some of the toluene present in the feed, is passed to an extractive distillation zone wherein said fraction in vapor phase is contacted with a solvent selective to the aromatics. The raffinate distillate from the extractive distillation zone is substantially free of aromatics and suitable for feed to an isomerization or paraffin alkylation process. On the other hand, the extract phase, comprising the selective solvent and dissolved aromatics, is directed to an extraction zone Where it contacts the bottoms from the fractionated hydrocarbon feed stream. The selective solvent removes from the bottoms the aromatics contained therein and thus a substantially paraffinic rafiinate is produced which contains little or no C and lighter paraftins. The extract from the extraction zone contains substantially all of the aromatics present in the hydrocarbon feed stream and these may be separated therefrom by known methods. If necessary, additional fresh solvent is fed to the extraction zone to remove anybenzene or toluene which may be re-extracted by the raifinate.

The instant process is advantageous in thatit produces an essentially aromatic-free C 'and C paraii in stream suitable for anisomerization or paraflin alkylation process. Likewise, the recovery of aromatics from the hydrocarbon feed stream is substantially increased, not only because the benzene taken overhead is recovered, but additionally because more severe operating conditions may be employed in the extraction zone and thus the heavy aromatics are recovered from the feed stream in greater quantities. The latter is possible because the low boiling paraffins, C and less, have been removed from the feed stream prior to the extraction step.

The exact nature and objects of the invention can be more fully understood from the following description and attached drawing which illustrates the preferred embodiment of the invention.

The FIGURE represents a simplified diagrammatic flow sheet of the preferred embodiment of the instant With reference to the figure a hydrocarbon feed containing aromatics, including heavy aromatics such as C3 and C and parafiins, including light parafiins. such as C and less, is directed through line 1 into distillation zone4 wherein the C and lighter hydrocarbons are taken overhead through line 2. The hydrocarbon feed stream may, for example, be the product from a reforming operation or process. Other high aromatic naphthas, such as cracked naphthas hydrogenated over .cobalt-molybdate catalysts or certain virgin naphthas are also suitable. The bottoms or residual hydrocarbons from zone 4 are passed into distillation zone 7 through line 3. Zone 7 is operated with an overhead temperature of from about 180 to about 380 F., preferably 180 to 300 F., and at pressures in the range of from about 0 to about p.s.i.g., preferably 0 to 50 p.s.i.g., in order that all the C and C paratfin hydrocarbons are taken overhead through line 5. These temperature and pressure conditions yield a cut point in the range of from about 150 F. to about 230 F. In so doing substantial amounts of benzene and some toluene will also be mixed with this C and C paraffin hydrocarbon stream. The hydrocarbons taken overhead from distillation zone 7 are maintained in the vapor phase and passed to an extractive distillation zone 19 wherein they are contacted with a solvent selective to aromatics, such as a diethylene glycol-water mixture, which is introduced to zone 19 through line 18. If desired, other nonolefinic refinery streams containing C and/or C parafiins may be combined with the overhead from zone 7 by means of line 27.

T o insure intimate contacting in the extractive distillation zone between the vapor phase C and C fraction and the liquid phase selective solvent, various means may be employed. A tower constructed in the same manner as an ordinary fractionating column of a bubble cap type comprises one effective form of apparatus for this purpose. A tower filled with suitable packing of refractory earthenware, glass, etc., is also an efiicient means for insuring sufiicient contact between the liquid and gas phases.

The extractive distillation column is operated at temperatures within the range of from about 150 to about 500 F, preferably '150 to 300 F., and at pressures in the range of from about to about 50 p.s.i.g., preferably 0 to 15 p.s.i.g., depending on the particular selective solvent employed. It will be understood that a temperature gradient exists across the extractive distillation column, the lower overhead temperature corresponding to the boiling point of the C C fraction or slightly higher and the higher temperature corresponding to the boiling point of the solvent-aromatic mixture withdrawn as bottoms. More specifically, the overhead temperature in the extractive distillation column may be within the range of about 150 to 250 F., preferably about 160 F, and the bottoms temperature within the range of about 250 to 500 'F., preferably about 250 to 350 F. In this extraction distillation tower the heat input, solvent to feed ratio and reflux ratio are adjusted in order to give the desired purity of products. Methods for optimizing these variables are well known to those skilled in the art.

The overhead product from extractive distillation zone 1119 is withdrawn through line 17 and contains C and C parafiins substantially free of aromatics. Thus the hydrocarbon stream in line 117 is suitable for feed to an isomerization or paraffin alkylation process. The extractphase from zone 19 is withdrawn through line 11 and comprises the solvent and dissolved aromatics.

The bottoms or residual hydrocarbon feed from distillation zone 7 is passed through line 6 to another distillation zone .10 wherein polymers and those high boiling aromatics which are readily removed from the feed are separated therefrom and withdrawn through line 9. The overhead product 8 from distillation zone therefore contains C and higher parafiins and those heavy aromatics, such as C and C which could not be readily separated from the residual hydrocarbon feed stream 6. The separation step in zone 10 is optional depending on the characteristics of the feed stream to be treated. The overhead product 8 is condensed and passed to extraction zone 16 wherein it contacts the extract phase from extractive distillation zone 19. The extraction zone is operated within the temperature range of from about 100 to 450 F. depending on the nature of the solvent, preferably 300 to 400 F., to allow maximum recovery of the heavy aromatics which require more severe operating conditions for their removal. Higher temperatures than would normally be used for the extraction of this range of aromatics are permissible in the extraction zone of the instant process because the C and C paraflins have been substantially completely removed from any feed stream entering this zone.

Although the particular manner in which the extraction of stream 8 is effected with the extract phase from the extractive distillation zone may be varied in accordance with known liquid-liquid extraction techniques, the figure diagrammatically represents the preferred extraction step wherein countercurrent flow in a vertical chamber is employed. The solvent and dissolved aromatics from zone 19 enter the upper portion of extraction zone 16 through line 11 and flow downwardly contacting the upwardly flowing hydrocarbons which entered through line 8. Fresh solvent may be added to the extraction zone above the inlet for line 11 to remove any benzene or toluene which may be re-extracted from stream 11 by the rafiinate product. Line 13 represents such a supply of fresh solvent. The raflinate product, which comprises parafiins of C and greater and is substantially free of aromatics, is withdrawn through line 12.

The extract phase from extraction zone 16 is removed by line 14 and passed to a flash tower 20 wherein a portion of the aromatics is taken overhead through line 21 and recycled to extraction zone 116. The remaining stream is passed through line 22 to a solvent recovery zone 24 wherein the aromatics are separted from the selective solvent by any known method. A preferred known method for separating the aromatics from the selective solvent is by distilling extract phase L14 in two stages as shown in the figure. Overhead streams 21 and 23 are suitable recycle for extraction zone 16, since any paraifins left in extract stream 14 will tend to distil-l overhead preferentially to the aromatics in zones 20 and 24 and therefore will be concentrated in stream 15. The aromatics are recovered as a side stream in zone 24 and will contain a substantially lower concentration of parafiins than may be found in extract stream 14.

Although the method discussed above for preparing extractor recycle 1'5 and recovering aromatic product 25 is preferred, other methods may be employed. For example, the hydrocarbons in stream :14, other than the solvent if it is a hydocarbon, may be removed by simple fractionation and a portion thereof recycled to the extrac tion zone 16. The remaining portion would be the aromatic product. The latter, however, would not be as pure as the aromatic product obtained by the preferred method.

In any event, the recovered solvent is recycled through line 26 to extractive distillation zone 119 through line 18 and extraction zone 16 through ine 16. The recovered aromatics are removed through line 25 to be processed as desired.

The effectiveness of the extractive distillation step in combination with fractionating the feed to remove all the C and C paralfinic hydrocarbons is illustrated in the following test. A hydrocarbon feed stream obtained from the C --l F. reformate fraction, having an analysis given in Table A below, was preheated to The tower was equipped with a feed inlet in the bottom portion thereof, a solvent inlet in the top portion thereof,

a reboiler, an automatic reflux divider and an overhead product outlet. Solvent selective to aromatics flowed downwardly, countercurrently contacting the vapor feed. The selective solvent was a diethylene glycol-water mixture containing 0.13 Wt. percent water and having a boil- 6 ethers of ethylene and diethylene glycols, such as the methyl-, ethyl-, propyland butyl-ethers of ethylene glycol and of diethylene glycol, the dimethyl-, diethyl-, dipropyl-, and dibutyl-ethers of diethylene glycol and mixed dialkyl ethers such as the methyl ethyl diethers, acetic ng point of 460.5 F. and speclfic gravity at 60/ 60 of ac1d,prop1on1c ac1d,butyr1c ac1d,benzaldehyde, cinnamal- 1.115. Bottom product, selective solvent plus dissolved dehyde, salicylaldehyde, anisicaldehyde, phenol, furfuralaromatics, could be removed from the reboiler as desired. dehyde, mono-, di-, and trinitrophenols, xylol, cresol, In Table B below the operating conditions, oil and feed nitrocresol, guaiacol, eugenol, isoeugenol, piperonal (3,4- rates and results are tabulated for a series of experimental methylene dioxybenzaldehyde), acetophenone, benzoruns: phenone, formamide, dimethyl, and diethyl formamide,

TABLE B Aromatic Removal by Extractive Distillation Run No 5 5 5 5 4 4 4 6 6 1 7 1 8 Period A B C D A C E C E B B Operating Conditions:

Reflux Ratio 2 2 2 2 2 2 2 3 4 1 1 Solvent: Feed Ratio, v./v 0 0 0 0 9. 5 8.3 9. 7 9.1 9.0 8. 8 13.1 Reboiler Temperature. F 172 155 133 190 275 325 375 275 250 242 230 Length of Period, Hrs 2.0 1. 5 2.0 2. 0 2. 5 2.0 3.0 3.0 2.0 1.0 2. 5 Wt. 01' Hydrocarbon feed, gm 603 378 610 611 269 246 318 329 222 112 200 Wt. of Solvent feed, gms 0 0 0 0 3,877 3,119 4, 684 4,590 3, 03s 1, 498 4,006 Bottom Product:

Total Wt. (Solvent and Hydrocarbon) gins 373 235 268 215 4, 011 3, 197 4, 779 4, 746 3,160 1, 534 4, 103 Wt. percent hydrocarbon (11.0.)-

100 100 100 100 2. 64 1. 93 1. 68 2. 3. 01 2. 97 2. 44 Wt. Percent C5 and C paraffin in 11.0- 0.0 1.4 2.2 Overhead Product:

we, 5 233 145 342 379 135 168 223 173 100 46 39 Vol. percent of Hydrocarbon feed 37. 7 38. 2 57. 7 68.6 62.2 75. 8 77. 7 64. 3 57. 7 59. 9 53. 8 Vol. percent Benzene 22 22 23 23 1 5 7 5 2. 8 0. 9 0.9

wat

1 In these runs the diethylene glycol solvent had a specific gravity at 60/60 of 1.117, a boiling point of 464.6 F. and 0.10 wt. percent er.

It is readily apparent from a comparison of runs 5A 5 sulfolane, and liquid substitution sulfolanes, such as 3- 5D to all the other runs in Table B that the use of an extractive distillation column substantially reduces the amount of benzene which would be lost in the overhead product. It is also worthy to note that the bottom product, which is directed to an extraction zone, will contain less than 3% of the C and C paraflin hydrocarbons fed to the extractive distillation zone and therefore will be substantially free of the paraflinic material which hinders the use of severe operating conditions in the extraction zone. It will be understood that any benzene which does not appear in the overhead product is dissolved in the selective solvent and becomes part of the bottom product which is directed to the extraction zone as previously described.

In any extractive distillation step there will be a need for experimental determination, which is within the ordinary skill of one in the art, of the solvent to feed ratios which will give optimum results and also the optimum reflux ratio at these solvent to feed ratios. For example, it has been determined that the solvent to feed ratio for the extractive distillation tower when employing a diethylene glycol-water mixture is within the range of from 1 to 20, preferably 2 to 10. If, for example, a solvent to feed ratio of 9 is employed the reflux ratio should be within the range of 3 to 1, preferably 2. Such a determination of optimum extractive distillation conditions is within the ordinary skill of one in the art and forms no part of the instant process. Relatively high solvent to feed ratios may be employed in the extractive distillation zone with little increased operational cost, since the solvent requirements for the extraction zone will normally be considerably higher than those of the extractive distillation zone. Although the instant invention has been described in particularity with the use of diethylene glycol-water solvent, many other solvents may be employed by one skilled in the art in practicing the invention. Specific examples of such solvents are: butanol, isobutanol, pentanol, hexanol, glycol, glycerol, diethylene glycol, dipropylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol (molecular weight 200400), alkyl methyl sulfolane and 2,4-disulfolane.

It will be understood that the instant invention may be incorporated into a larger system wherein the feed material may first be passed to a reformer and the product therefrom passed to distillation zone 4. The recovered butanes and C and C paraffin hydrocarbons may be combined and passed to an isomerization and/ or paraifin alkylation step to produce isoparaflins suitable for gasolines. The quantity of aromatic-free paraifins for such processes may be augmented by adding other non-olefinic refinery streams to the extractive distillation zone feed. The recovered aromatics may likewise be sent to known aromatic fractionating and purification steps wherein the benzene, toluene and xylene are separated and used as desired. Furthermore, the whole or a high boiling fraction of the recovered rafiinate from the extraction zone may be recycled to the reformer.

What is claimed is:

1. In a process for separating paraflins and aromatics from a hydrocarbon stream containing C -C parafiins and C C aromatics, the improvement comprising fractionating said hydrocarbon stream at a cut point of from about to about 230 F. to remove all the C aromatics and C and C paraflins therefrom, countercurrently contacting said C aromatics and said C and C paraffins in vapor phase with a liquid solvent selective to aromatics in an extractive distillation zone, removing from said extractive distillation zone a vapor rafiinate and a liquid extract phase, countercurrently contacting said liquid extract phase in an extraction zone with the residual hydrocarbons from the fractionation step, Withdrawing from said extraction zone a substantially aromatic-free raflinate and an aromatic-containing second extract phase, and recovering from said second extract phase the aromatics contained therein and wherein said C aromatics are benzenes.

2. The process as described in claim 1 wherein fresh solvent selective to aromatics is countercurrently contacted in the extraction zone with said residual hydrocarbons, said fresh solvent entering the extraction zone upstream of the flow of said extract phase.

3. The process as described in claim 1 wherein the extractive distillation zone is maintained at temperatures Within the range of from about 150 to about 500 F. 5 and the extraction zone is maintained at a temperature Within the range of from about 100 to about 450 F.

4. A process according to claim 1 wherein said hydrocarbon stream is the product from a reforming operation.

5. A process according to claim 1 wherein said hydro- 10 carbon stream is a highly aromatic naphtha.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN A PROCESS FOR SEPERATING PARAFFINS AND AROMATICS FROM A HYDROCARBON STREAM CONTAINING C5-C8 PARAFFINS AND C6-C9 AROMATICS, THE IMPROVEMENT COMPRISING FRACTIONATING SAID HYDROCARBON STREAM AT A CUT POINT OF FROM ABOUT 150* TO ABOUT 230*F. TO REMOVE ALL THE C6 AROMATICS AND C5 AND C6 PARAFFINS THEREFROM, COUNTERCURRENTLY CONTACTING SAID C6 AROMATICS AND SAID C5 AND C6 PARAFFINS IN VAPOR PHASE WITH A LIQUID SOLVENT SELECTIVE TO AROMATICS IN AN EXTRACTIVE DISTILLATION ZONE, REMOVING FROM SAID EXTRACTIVE DISTILLATION ZONE A VAPOR RAFFINATE AND A 